Various methods and devices exist for measuring orientation (e.g., pitch, yaw and roll) of a vehicle. Pitch, denoted as θ, is the angle between a true vertical axis as defined by gravity and a vehicle-fixed substantially vertical axis, denoted as R. R may be defined, for example, by a seat back or another vehicle-fixed structure. Pitch sensing devices include accelerometers, inclinometers, gyroscopes, pendulous reference sensors, also known as tilt sensors, and distance probes.
Single axis state estimators (SASEs), can provide an estimate of the pitch of a vehicle over regular surfaces under most expected conditions. The pitch state becomes inaccurate where the vehicle makes certain changes in direction, such as, turning the vehicle around on a sloped surface. In such a situation, a SASE may indicate that the vehicle still has a positive pitch even though the vehicle is facing downward where the inclinometer should indicate a negative pitch. The sensors saturate if the pitch suddenly changes so that the pitch rate is above the temporal threshold at which the device may measure change.